Millimetre-wave illumination source

ABSTRACT

An illumination source of predominantly non-directional and incoherent millimeter-wave radiation for illuminating an area for passive millimeter-wave imaging comprises a container with at least a partly reflective internal surface and a plurality of exit apertures and a primary source of millimeter-wave radiation for emitting millimeter-wave radiation into the container. The primary source and the container are arranged so that a proportion of the millimeter-wave radiation emitted by the source undergoes reflection within the container before being emitted through the apertures, such that the different paths lengths are at least equal to the coherence length of the radiation. This is facilitated if the bandwidth of the radiation is preferably at least 1 GHz. The container may be a box in which a waveguide is used to couple radiation from the primary source into the box. Alternatively, the container may be formed from a mesh and the plurality of holes is provided by the holes in the mesh.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an illumination source for passivemillimeter-wave imaging.

2. Description of the Art

Many available imagers for security applications can produce imageswhich enable an operator to readily detect concealed weapons such asguns and knives, which are hidden for example within a person's clothingor baggage. Clothing and baggage materials are virtually transparent toat least some of these known imagers and this may be advantageous whenmetallic objects are solely of interest, since they will not be obscuredby non-metallic material.

This does mean, however, that such imagers are not capable of providingrecognisable images of passive non-metallic objects such as plastics,ceramics and explosives, which nowadays are often of more interest.

Millimeter-wave imaging addresses this problem. In outdoor passivemillimeter-wave imaging high contrast is provided in the generated imageby cold sky illumination. Depending on the geometry of the viewed scene,materials such as metals can reflect this illumination towards theimager, appearing cold and exhibiting a high contrast against thegenerally warm background. In addition, however, it is possible to usemillimeter-wave imaging for detecting passive non-metallic objects, andthis technique also allows the remote and covert scanning of suspects.

Passive millimeter-wave imaging can be accomplished indoors but the lackof sky illumination means that the main source of contrast is now theactual temperature difference between objects. This contrast will be ofthe order of 10K, which is an order of magnitude less than what can beexpected in outdoor imagery.

Another alternative is to use an artificial source of millimeter-waveradiation, to illuminate the area being imaged in order to improve thecontrast in the generated image. The relatively long wavelength ofmillimeter-wave radiation means that many reflections from visibly roughor dull surfaces are specular in nature in the millimeter-wave portionof the electromagnetic spectrum, i.e. many visibly rough or dullsurfaces behave similarly to a mirror to millimeter-wave radiation. Thiseffect is noticeable when a person being imaged is illuminated from asmall source. In this situation the person does not appear uniformlywarm in the generated image, but instead warm glints appear on the bodyof the person where specular reflection of the source from the body isincident on the aperture of the imager. This effect makes the generatedimage difficult to interpret in real time, particularly in real time.

This problem can be overcome by locating large areas of radar absorbentmaterial, for example over the walls, ceiling and floor of an indoorarea in which passive millimeter-wave imaging is to be done and heatingor cooling the radar absorbent material to a temperature that isdifferent from the ambient temperature of the objects in the indoor areathat are being imaged. Alternatively, large area portable panels ofheated or cooled radar absorbent material can be set up in the indoorarea in which imaging is to take place. This approach requires a lot ofenergy to heat or cool the large area of radar absorbent material anddoes not lend itself to portability.

It would therefore be desirable to provide a non-directionalillumination source of millimeter-wave radiation, where the radiationalso has a low degree of coherence, or no coherence, and which hasrelatively low power consumption. It would also be desirable to providesuch an illumination source in the form of a uniformly radiatingsurface. Ideally the radiating surface would appear to have the samebrightness at all angles of observation, so approximating a black bodyradiator.

SUMMARY OF THE INVENTION

In a first aspect the present invention provides an illumination sourceof millimeter-wave radiation for illuminating an area for passivemillimeter-wave imaging, the source comprising a container with at leasta part of its interior formed from reflective material and having aplurality of exit apertures; and a primary source of millimeter-waveradiation for emitting millimeter-wave radiation into the container;

-   -   wherein the primary source and the container are arranged so        that at least a portion of the millimeter-wave radiation emitted        by the primary source undergoes reflection within the container        and each said aperture receives radiation from the source via at        least two paths of different respective lengths.

Preferably the said at least two paths differ by more than the coherencelength of said radiation, and since coherence length is dependent onbandwidth (a function of propagation speed over bandwidth), preferablythe radiation has a bandwidth of at least 1 GHz. This provides acorrespondingly low coherence length of around 30 cm, and higherfrequencies give correspondingly shorter coherence lengths.

Indeed, from another viewpoint, the requirement that the said at leasttwo paths differ by more than the coherence length of said radiation ismost easily met when the radiation has a large bandwidth. Furthermore,the millimeter-wave imager that would be looking at the illuminatedscene is likely to have a bandwidth in the GHz range, and theilluminating source would need to cover all of this bandwidth in orderto make best use of the sensitivity of the imager.

Furthermore, using a wide bandwidth is one way of reducing the chancesof dielectric layers not being visible due to interference effects.

Accordingly in a second aspect the invention provides an illuminationsource of millimeter-wave radiation for illuminating an area for passivemillimeter-wave imaging, the source comprising a container with at leasta part of its interior formed from reflective material and having aplurality of exit apertures, and a primary source of millimeter-waveradiation for emitting millimeter-wave radiation with a bandwidth of atleast 1 GHz into the container;

-   -   wherein the primary source and the container are arranged so        that at least a portion of the millimeter-wave radiation emitted        by the primary source undergoes reflection within the container        before being emitted through the apertures.

Preferably the bandwidth is at least 2 GHz, more preferably at least 10GHz, and in some applications the bandwidth is 40 GHz or more. Relevantsources of radiation might be noise sources or amplifiers that amplify anoise source or load.

The source may be located entirely within the container, which providesa more self-contained arrangement. However, care then needs to be takento ensure oscillation of the system cannot occur, for example if some ofthe radiation inside the container is fed back to the source via a leakin the structure (such as at a waveguide join) and re-amplified, whichcould well give rise to a change in the source output and bandwidth andadversely affect the overall performance of the panel. One way ofpreventing radiation getting to the source components is to enclose themseparately within the container with only the radiating apertureaccessible.

One advantage of providing a source completely within the container isthat it is then much easier to tile a plurality of suitably shapedcontainers together to synthesise a larger illumination source withoutsignificant gaps.

Alternatively, the source or parts thereof may be outside the containerand coupled thereto e.g. by a waveguide, a horn or a diffusive devicesuch as a leaky waveguide. A diffusive device has the benefit offacilitating spreading of the primary input radiation to more than onelocation within the container. An alternative form of diffusive devicewould be smaller version of the overall panel itself contained withinthe container. Preferably the device is selected such that it enablescontrol of the level of radiation fed into the container.

The primary source is preferably arranged to act essentially as a pointsource, for example by forming an input aperture to the volume of thecontainer (whether from outside the container or from a source whollywithin the container) which has a size approximating to the operatingwavelength of the source.

Preferably the apertures have a width approximating to half theoperating wavelength of the illumination source, so that the radiationemitted from the apertures spreads out with a solid angle approaching 2πsteradians. As noted later, the beam patterns depend on polarisation andthe direction of observation, resulting In different E and H planepatterns, according to standard aperture theory. Care needs to be takenin selecting the hole dimension(s) insofar as when the dimension issmall relative to wavelength the transmission through the hole varies asa high power of the dimension. This means that while reducing holedimension(s) may improve the radiation pattern, in particular increasingits width or angular range, the apparent or observed millimeter-wavetemperature of the radiating surface (i.e. the surface incorporating theapertures) may fall to an undesired extent.

The apertures may all have the same shape and size, or the shape and/orsize may vary (for example between 2 or 3 different discrete shapes orsizes, or there may be a continuous variation in shape/size across thepanel) to alter the overall pattern of radiation from the radiatingsurface.

In one embodiment the container is a box and a waveguide is used tocouple millimeter-wave radiation from the primary source into the box.In another embodiment at least part of the container is formed from amesh and the plurality of holes is provided by the plurality of holes inthe mesh.

The reflections that the radiation from the primary source undergoesbefore being emitted through the apertures in the container mean thatsome radiation from the source travels further within the container thanother radiation before being radiated at a particular aperture. Thishelps to decrease the coherence of the radiation emitted from the panelas a whole and from adjacent apertures in particular. Preferably over50%, more preferably at least 75% and even more preferably at least 90%of the millimeter-wave radiation emitted by the primary source undergoesreflection within the container before it is emitted through theapertures.

The incoherence of the emitted radiation may be further promoted whereat least a part of the interior of the container is formed from roughreflective material so that incident light on the rough reflectingmaterial is reflected in different directions. In this context “roughreflective material” means reflective material with discontinuities of asize approximating or greater than the wavelength of the radiation.Also, by making the apertures small the emitted radiation pattern isbroad, and therefore relatively non-directional (although it still has adefinite maximum intensity direction).

Incoherence and non-directionality of the radiation from the aperturesare two of the requirements noted above for good indoor illumination forpassive millimeter-wave imaging and they can be achieved by the presentinvention while consuming much less power than is required in theheating or cooling of large areas of radar absorbent material.

It is also very desirable that the power of the millimeter-waveradiation emitted from each aperture in the container is similar so asto provide a uniformly radiating surface when viewed with a relativelylow resolution (so as not to resolve individual apertures). This can beachieved in a variety of ways, such as by the careful arrangement of thepattern of the apertures formed in the container, by the use of at leastone reflective baffle and/or at least one region of millimeter-waveabsorbing material within the-container or by covering the aperturesemitting the highest power millimeter-wave radiation by a partiallyreflective dielectric element or by a partially absorbing material.

In general, uniform radiation is assisted considerably by ensuring thatthe emitted radiation has undergone many reflections within thecontainer, for example by making the internal surface of the radiatingsurface more highly reflective and by careful control of aperture sizeand shape.

In addition the polarisation of the radiation emitted by the primarysource relative to the radiating surface can be important. The E and Hplane beam patterns of the apertures have an effect within the box aswell, and it has been shown to be desirable to choose that polarisationthat is less likely to be directly radiated, i.e. the one thatcorresponds to the H plane of the aperture, and is parallel to the planeof the radiating surface rather than perpendicular to it.

A further desirable feature would be that the radiating surface appearsto have the same brightness at all angles of observation, soapproximating a black body radiator. To this end, as will be explainedin greater detail later, a low loss dielectric material may be locatedat or immediately adjacent the apertures to intercept radiation passingthrough the apertures. Conveniently the dielectric material may take theform of a sheet of material over the exterior of the radiating surface,but other arrangements are possible.

A further use of a sheet of dielectric material over the radiatingsurface is to control the direction of the radiation leaving theapertures. For example, the sheet may be wedge-shaped so as to refractthe main radiation direction away from the normal; or the sheet may bestepped to act as a sort of Fresnel lens, for focusing or redirectingthe radiation from the apertures depending on the pattern of the steps(e.g. a circular stepped pattern could be used for focusing as in aFresnel lens, but a linear stepped pattern could act as a wedge). Ineach case the sheet has a non-uniform thickness, and is preferableplaced on the outside of the radiating surface.

Sources of GHz frequency radiation comprising a container with at leasta part of its interior formed from reflective material and having aplurality of exit apertures; and a primary source of millimeter-waveradiation for emitting millimeter-wave radiation into the container, inwhich the primary source and the container are arranged so that at leasta portion of the millimeter-wave radiation emitted by the primary sourceundergoes reflection within the container before being emitted throughthe apertures are known. For example, UK Patent Application GB 2 233 502A (Arimura Giken KK) discloses slot antennae in which input radiationenergy at 12 GHz for one polarisation and 14 GHz for the orthogonalpolarisation is reflected into a rectangular waveguide having a slottedface, residual radiation being absorbed at the far end of the guiderather than being available for reflection. The description refers topower radiating from the slots “in equiphase”, and to arranging the slotspacings according to the operative wavelength. There is a relateddisclosure GB 2 208 969 A (also Arimura Giken KK).

By contrast, the present invention is concerned with the provision of abroadband millimeter-wave illumination source and/or the provision of asource which radiates predominantly incoherent illumination.

The present invention extends to an array of illumination sourcesaccording to the first or second aspects of the invention, and to animaging system comprising a millimeter-wave imager for imaging apredetermined region, and an array of illumination sources according tothe first or second aspects of the invention arranged to illuminate saidregion.

In certain instances, it may merely be necessary to arrange theillumination sources either side of the region, for example in twoopposed straight lines. However, unless the radiation from each sourceis uniform, some sources will contribute more radiation to the localregion than others. Therefore, when the sources have a direction inwhich there is a maximum amount of radiated energy (the principalradiation direction) it is preferred to arrange the sources with theirprincipal radiation directions generally directed towards the same morelocalised region. This can be done by inclining the sources relative toeach other, and/or by arranging that the radiation from at least onesource is refracted or steered as it leaves the radiating surface, forexample as previously indicated by providing a dielectric wedge orlinear stepped pattern on the radiating surface, for example.Additionally or alternatively, the radiation from one or more radiatingsurfaces may be “focused” for example by providing a dielectric layerthereon having a circular stepped pattern much as in a Fresnel lens.

The present invention will now be described by way of example only withreference to the accompanying figures in which:

DESCRIPTION OF THE FIGURES

FIG. 1 shows a cross-section through a first embodiment of a source forpassive millimeter-wave imaging according to the present invention;

FIG. 2 shows a cross-section of a source for passive millimeter-waveimaging generally similar to that shown in FIG. 1, but with an L-shapedcross-section;

FIG. 3 shows another embodiment of a source for passive millimeter-waveimaging according to the present invention employing a wire mesh;

FIG. 4 is a plot of the variation of the observed or apparenttemperature (in arbitrary units) of the radiating surface withobservation angle for the E and H plane components for an illuminationsource such as that of FIG. 1;

FIG. 5 is a drawing of a container of a modified illumination sourceaccording to the invention for explaining the effect of adding a sheetof low loss dielectric material over the radiating apertures;

FIG. 6 is a theoretical plot of the variation of the observed orapparent temperature (in arbitrary units) of the radiating surface withobservation angle for the E and H plane components for the container ofFIG. 5;

FIG. 7 is a measured plot of the variation of the observed or apparenttemperature (in arbitrary units) of the radiating surface withobservation angle for the E and H plane components for a containersimilar to that of FIG. 5;

FIG. 8 shows in schematic form a first imaging system incorporating aplurality of illumination sources according to the invention;

FIG. 9 shows in schematic form a second imaging system incorporating aplurality of illumination sources according to the invention and

DESCRIPTION OF A PREFERRED EMBODIMENT

In the arrangement in FIG. 1 millimeter-wave electromagnetic radiationis generated in a millimeter-wave source 2, such as an amplified noisesource and is coupled into a metal box 4 using waveguide 6. The source 2will generate millimeter-wave radiation associated with a temperaturewhich is either much higher or much lower than the ambient temperatureof the objects in the area being imaged. In this way higher contrast canbe introduced into the generated image. The metal box 4 has an internalreflective surface which may, for example, have a reflectivity of 0.5 orgreater at the operating wavelength. At least one side of the box 9 isformed with an array of through holes or apertures 8, which may forexample be circular, six of which (8 a-8 f) can be seen in FIG. 1.Normally the array will be two-dimensional, although the use of aone-dimensional or linear array also falls within the scope of theinvention. The waveguide 6 has an aperture at its end remote from thesource 2 having a diameter approximating the operating wavelength of theradiation generated by the source 2. Thus, the radiation from thewaveguide 6 spreads out on entry into the box 4 With a solid angleapproaching 2π steradians, so that the end of the waveguide 6effectively acts as a point source.

Some of the radiation emitted from the end of the waveguide into the box4 will travel directly to one of the holes 8 but the majority of theradiation will undergo at least one reflection at the internal surfaceof the box 4 before reaching a hole. The holes 8 each have a diameterapproximating to half of the wavelength of the radiation generated bythe source 2 and so the radiation emitted from the holes will spread outwith a solid angle approaching 2π steradians. This is shown for exampleby the wavefronts 10 of the radiation emitted from the hole 8 a.

The millimeter-wave radiation can have a coherence length of the orderof several tens of millimeters depending on its bandwidth. Thedimensions of the box 4 are chosen so that when internal reflections aretaken into account the path length differences of radiation travellingbetween the source and each hole 8 will tend to be equal to or greaterthan its coherence length, thus ensuring that interference effects fromthe millimeter-wave radiation emitted by the holes 8 will beinsignificant. Also, due to the small size of the holes 8 the radiationemitted from the holes will be non-directional. The side of the box 4 inwhich the holes 8 are formed itself forms a radiating panel or radiatingsurface which can be part of a one or two-dimensional array of suchradiating panels located at or around an area which requiresmillimeter-wave illumination.

For satisfactory illumination, preferably all reflections off thesurface of the object being observed which are seen by a millimeter-waveimager observing the illuminated scene should originate at anilluminated panel, either directly or via other reflections. Eachilluminating panel preferably has a constant average radiationtemperature across its surface and will be capable of radiating into alarge solid angle. Where the illuminating panel makes use of a pluralityof radiating apertures, those apertures should preferably besufficiently close together that they cannot be resolved by the imagingsystem when used for imaging an object illuminated by the panel (i.e.the panel has a uniform appearance when viewed as reflected by theobject being imaged. The imaging system is not normally focused on thepanel itself.) This will ensure that the observed radiation temperatureis approximately constant across the surface of the panel. Similarly,gaps between adjacent panels can be accommodated provided they are notresolvable by the imaging system when used to image an objectilluminated thereby.

Where a non-portable millimeter-wave source is required, the box 4 couldbe formed with holes in only one of its sides and the walls, floorand/or ceiling of an indoor area where the imaging is to take place canbe at least partially tiled with a plurality of such sides of suchboxes. Using an arrangement of such boxes according to the presentinvention will use significantly less energy than would be required toheat or cool the equivalent area of radar absorbent material.

Where a portable version is required, one or more such boxes with holesformed a suitable number of its sides could be located at or around thearea where the imaging is to take place. While the originally intendeduse of these panels was for indoor locations, they could also be usedoutside to provide illumination for other passive millimeter wavesystems, including those at outdoor locations.

Ideally, the radiation intensity from each aperture has the same value.If it is found that the intensity of radiation is higher from someholes, for example holes closer to the source 2, then partiallyreflective dielectric could be used to cover these holes, for examplethe dielectric 16 located over the hole 8 a in FIG. 1 which is closestto the source 2. Alternatively or additionally, some absorbing material,such as radar absorbent material, could be fitted over part of theinternal or external surface of the box.

The pattern of holes used can be adjusted to make the illumination fromthe box 4 as uniform as possible, so to produce a more uniform averagetemperature profile across the surface of the box containing the holes8. In addition or alternatively, reflecting baffles, dielectrics orabsorbers could be located within the box or on the internal surface ofthe box in order to alter the radiation pattern generated by the box,and/or the primary source location or aperture type may be appropriatelyadjusted.

The embodiment of FIG. 2 shows that the box need not be of simplerectangular cross-section. As shown, the box 4 has an L-shapedcross-section with the source 2 and waveguide 6 located at one end ofthe L and a optionally reflecting baffle 12 located at the bend in theL, but other shapes of box could be employed. Where present thereflecting baffle 12 could have a rough reflecting surface in order tofurther decrease the coherence of radiation emitted from the box 4. By“rough reflecting surface” is meant a reflecting surface withirregularities on a scale equal to or greater than the wavelength of theradiation. The radiating holes are formed for example in the end 4 a ofthe box 4 where not obscured by the baffle 12. In this embodiment, radarabsorbent material absorbing material 14 is fitted over selected areasof the interior of the box to render the outputs of the apertures 8 moreuniform.

A further embodiment of the present invention is shown in FIG. 3, inwhich the illuminating source for indoor passive millimeter-wave imagingcomprises a millimeter-wave source 22 and a container in the form of adome 24 made of mesh which has a flat base 30 with a reflecting uppersurface. The source 22 is located at the center of the dome 24 and hasan aperture 26 on its upper surface. The size of the aperture 26 is ofthe order of half of the operating wavelength of the source so that thesource radiates over a wide solid angle. The underside of the mesh whichforms the dome is reflective and may be formed from reflective metal.The base 30 may also comprise a metal sheet with a reflective uppersurface. The spacing of the strips or wires making up the mesh ispreferably less than half the operating wavelength of the source 22. Dueto this spacing of the strips or wires making up the mesh a largeproportion of the radiation will be reflected by the underside of themesh at least once before it is emitted through the mesh. The spacing ofthe strips or wires forming the mesh can be used to control theproportion of the radiation reflected from the mesh before it is emittedthrough the mesh. The architecture of the dome 24 will cause themajority of radiation from the source 22 to be reflected a large numberof times within the dome which will destroy the coherence of theradiation from the source. Using a rough reflecting surface as the uppersurface of the base 30 or baffles within the dome can further reducecoherence.

Thus the mesh dome 24 radiates as a spatially incoherent source in alldirections with a substantially uniform intensity.

Again, however, if necessary, areas of the container may be covered withradar absorbent material, for example on the base 30, and/or selectedareas of the mesh may be covered with a partially reflecting dielectricsheet.

It should be noted that the sources of FIGS. 1 to 3 may be arranged toemit both horizontally and vertically polarised radiation. This can beused to further improve contrast in a millimeter-wave imaged scene ifimages are taken separately using the differently polarised radiationand then processed.

As noted previously, a further desirable feature would be that theradiating surface appears to have the same brightness at all angles ofobservation, so approximating a black body radiator.

For this to occur, all the individual radiating elements (apertures)would need to have a cos theta radiation pattern, dropping to zero at a90 degree incidence angle. Such a pattern would cancel out the 1/(costheta) increase in area, as seen by a beam of constant solid angle, asthe angle of observation of a notional infinite surface moves away fromnormal.

However, the radiation pattern from circular or rectangular holes doesnot follow this law, and, indeed, the E and H patterns are not equal, sothat the surface will appear to be at different temperatures dependingon the observation direction. This is shown in FIG. 4, which is amodeled curve for a circular hole, found to agree closely withexperiment. Thus in reality the apparent surface temperature drops withincreasing angle in the H plane, curve 32, because the H plane radiationfrom a circular hole is narrower than the ideal cos theta pattern, butrises in the E plane, curve 34, because the E-plane radiation is widerthan the ideal. The ideal behaviour would be the same horizontal linefor both E and H plane radiation.

If low loss dielectric material 18 such as polythene is placed over eachaperture 8, FIG. 5, conveniently as a continuous sheet but optionally asindividual components for each aperture, the emitted radiation from theaperture will be refracted, bending rays away from the normal. Thus someradiation 19 emitted at a large angle to the panel normal which wouldhave been emitted is now totally internally reflected in the dielectric,and is either passed back into the container 4 or, more probablyretained within the dielectric 18 and eventually absorbed or emittedalong the edges of the panel or over its entire surface at a differentangle to the normal. Radiation 20 that travels through the dielectricsheet at an angle of incidence between its critical angle (ray 21) andits normal is now spread out over the full 90 degree range as it leavesthe aperture.

The effect on the emitted radiation pattern is shown in FIG. 6, and itwill be seen that the E-plane radiation pattern 36 is now very muchflatter, and the H pattern 38 is improved, although it still falls withincreasing angle.

A similar effect is shown in FIG. 7 for a panel with 5 mm diameter holeson a 50 mm square pitch. The arrows E and H respectively indicate theeffect on the apparent or observed temperatures of the E and Hcomponents on adding a 10 mm thick polythene sheet to the surface.

FIG. 8 shows an imaging arrangement comprising a millimeter-wave camera40 for viewing a subject 42 in a local imaging region between opposedrectilinear lines of illumination sources 44 according to the invention.Although the sources could be used without any modification, as shownthe sources more remote from the subject are provided with dielectricwedges on their radiating surfaces so as to deflect by refraction theradiation closer to the subject.

FIG. 9 shows an alternative imaging arrangement in which the more remotesources 44 are inclined relative to the closer sources so that thenormal to the radiating surface of each source is generally directedtowards the subject. Naturally, both a geometrical arrangement such asin FIG. 9 and beam deflection for example as in FIG. 8 may be employedconjointly to obtain the optimum illumination of the subject.

Where there is a plurality of sources 44, they may be arranged to beindividually or separately controlled, for example to turn on each one,or each group of sources, independently of each other. This will alterthe radiation pattern received by the subject, and may show up‘shadowing’ effects of objects and provide useful additionalinformation.

It should be noted that, especially if a panel is being reflected insomething as well as directly illuminating a person or object, theprincipal radiating direction may not be the direction of the person orobject. Similarly, where as in FIG. 9 there is a plurality of relativelyinclined sources, they may alternatively be arranged so that theirprincipal radiating directions converge at a locality which is notcoincident with the position of the object.

Although FIGS. 8 and 9 show a plurality of sources 44, a single sourcecould be used in some instances. Whether there is one source, or aplurality, the pattern of radiation at the subject may be furtheradjusted or controlled by providing one or more additional reflectorsexternal to the source(s), such as a mirror or diffuser.

A panel may be strengthened (e.g. for standing on) by using dielectricor metal materials within it, without adversely affecting itsperformance.

1. An illumination source of incoherent millimeter-wave radiation forilluminating an area for passive millimeter-wave imaging comprising acontainer with at least a part of its interior formed from reflectivematerial and having a plurality of exit apertures; and a primary sourceof millimeter-wave radiation for emitting millimeter-wave radiation intothe container via at least one exit port wherein an exit port associatedwith the primary source is adapted to direct at least some of theradiation towards at least a first reflective surface that is itselfarranged to reflect at least some of the radiation towards a secondreflective surface or second region of the first reflective surface, theexit apertures allowing radiation to escape from the container.
 2. Anillumination source according to claim 1 wherein said at least two pathsdiffer by more than the coherence length of said radiation.
 3. Anillumination source according to claim 1 wherein the radiation has abandwidth of at least 1 GHz.
 4. An illumination source according toclaim 1 wherein a portion of the radiation is not reflected before beingemitted through the apertures.
 5. An illumination source according toclaim 1 wherein the majority of the millimeter-wave radiation emitted bythe primary source undergoes reflection within the container beforebeing emitted through the apertures.
 6. An illumination source accordingto claim 1 wherein at least one reflective baffle is located within thecontainer.
 7. An illumination source according to claim 1 wherein atleast one region of millimeter-wave absorbing material is located withinthe container.
 8. An illumination source according to claim 1 wherein atleast one of the exit apertures is covered by a partially reflectivedielectric element.
 9. An illumination source according to claim 1wherein the primary source is substantially a point source.
 10. Anillumination source according to claim 1 wherein the source is coupledto the container by a waveguide.
 11. An illumination source according toclaim 1 wherein the source is located within the container.
 12. Anillumination source according to claim 1 wherein the container is a box.13. An illumination source according to claim 1 wherein at least part ofthe container is formed from a mesh and the plurality of apertures areformed by the holes in the mesh.
 14. An illumination source according toclaim 13 wherein the container comprises a dome of mesh located over abase having a reflective upper surface.
 15. An illumination sourceaccording to claim 13 wherein the mesh is made from metal wire.
 16. Anillumination source according to claim 1 wherein the apertures have awidth approximating to half the operating wavelength of the illuminationsource.
 17. An illumination source according to claim 1 wherein a lowloss dielectric material is located at or immediately adjacent theapertures to intercept radiation passing through the apertures.
 18. Anillumination source according to claim 17 wherein the low lossdielectric material is in the form of a sheet on a surface of thecontainer incorporating the apertures.
 19. An illumination sourceaccording to claim 17 wherein the low loss dielectric material has anon-uniform thickness to control the direction of the radiation leavingthe apertures.
 20. An imaging arrangement comprising a millimeter-waveimager for imaging a local region and at least one illumination sourceaccording to claim 1 arranged for illuminating said region.
 21. Animaging arrangement according to claim 20 and including a plurality ofsaid sources.
 22. An arrangement according to claim 21 wherein thesources have a preferred principal radiation direction and are arrangedso that their principal radiating directions converge.
 23. An imagingarrangement according to claim 20 and including means for selectivelycontrolling one or more said sources whereby to alter the radiationpattern received at said local region.
 24. An arrangement according toclaim 20 and also including at least one passive millimeter-wavereflector for altering the pattern of radiation received by the saidlocal region from the said source.
 25. An arrangement according to claim24 wherein said passive device is a mirror or diffuser.
 26. Anillumination source of incoherent millimeter-wave radiation forilluminating an area for passive millimeter-wave imaging comprising acontainer with at least a part of its interior formed from reflectivematerial and having a plurality of exit apertures; and a primary sourceof millimeter-wave radiation for emitting millimeter-wave radiation witha bandwidth of at least 1 GHz into the container via at least one exitport wherein an exit port associated with the primary source is adaptedto direct at least some of the radiation towards at least a firstreflective surface formed from the reflective material, the exitapertures allowing radiation to escape from the container.
 27. Anillumination source according to claim 26 wherein at least one of theexit apertures is covered by a partially reflective dielectric element.28. An illumination source according to claim 26 wherein the apertureshave a width approximating to half the operating wavelength of theillumination source.
 29. An illumination source according to claim 26wherein a low loss dielectric material is located at or immediatelyadjacent the apertures to intercept radiation passing through theapertures.
 30. An illumination source according to claim 29 wherein thelow loss dielectric material is in the form of a sheet on a surface ofthe container incorporating the apertures.
 31. An illumination sourceaccording to claim 30 wherein the low loss dielectric material has anon-uniform thickness to control the direction of the radiation leavingthe apertures.